Green Roofs – Digging Deeper

August 14, 2009

This month, I met with a friend to discuss the fit-out of a relatively non-descript office space. What the space lacks in amenities, it makes up for in potential, with planned windows opening to views of historic Newburyport in one direction and an expanse of marshland in the other. Adjacent to this space is a large flat roof that is nearly at the same height as the office’s floor -a great opportunity for a roof garden or a green roof.

Green roofs, also called living or planted roofs, are systems of living plants and vegetation installed on an existing or new structure. Popular in Europe for decades, the technology has seen continued improvement, making green roofs available in and appropriate for nearly all climates and areas of the United States, even in New England!

Chicago City Hall (photo by Roofscapes)

I relocated to the east coast from Chicago in 2002, just as the greening of the city was taking off. The then and still-reigning king of Chicago, Mayor Daley, was inspired by a trip to Germany in the late 90’s. The rest is Chicago green building (and green roof) history – In 2001, the first green roof in Chicago was installed on City Hall. Mayor Daley and the city’s efforts have been successful through mandates and incentives for green roofs and other green building features on public buildings and new developments that receive money from the city. Chicago now boasts more than 600 green roofs, or 560,000sf of green roof – my favorite view over martinis from the Signature Room in the Hancock Tower will never be the same!

Holyoke College, Holyoke, MA (photo by Roofscapes)

Boston Children’s Museum (photo by BCM)

As part of my research for this project, I sought local experts.Through Roofscapes, a green roofing product manufacturer, I made contact with two Massachusetts local green roof design/builders: Apex Green Roofs in Somerville and Earth Our Only Home in Boston. Both have vast experience in the construction of public/commercial and residential green roofs. Their websites offer loads of information about green roofs and photos of their work – we’re happy to have them as local resources.

Green Roofs – Some Basics:

I found clear and concise information on green roofs from Toolbase Design and Construction Guide. The following detailed descriptions are mostly gleaned from that site. I’ve also included other useful links at the bottom of the post.

What are the Benefits?

The added mass and thermal resistance of green roofs reduces the heating and cooling loads of the building. These systems reduce the ambient temperature around the roof, decreasing the building’s urban heat island effect; reduce the ambient temperature of the roof’s surface; and slow the transfer of heat into the building, reducing cooling costs. They also provide added insulation to the roof structure, reducing heating requirements in the winter.

Green roofs reduce stormwater runoff by absorbing and retaining the water in the soil medium for plant growth. The plants can filter pollutants and carbon dioxide from the air and rain water. These systems reduce rooftop temperatures and can reduce air and noise pollution. They also serve as living habitats for birds and other wildlife.

All green roof systems consist of four basic components: a waterproofing layer, a drainage layer, a growing medium, and vegetation. Some green roofs also include root retention and irrigation systems, but these are not essential. There is a wide variety of materials used for each component of the green roof system, depending on the chosen plants, type of system employed, climate, and underlying structure.

Waterproofing Layer – The waterproofing membrane is a critical component of the system and should include a root barrier to ensure the underlying roof surface is not compromised. If the weatherproofing material is not root-resistant, an additional layer must be applied to serve this purpose.

Drainage Layer A drainage layer is required to adequately distribute water and prevent pooling. To minimize the weight of the system, drainage layers are often made from plastic or rubber, but may also be made of gravel or clay. The drainage layer may or may not include filter media to ensure aeration.

Growing Medium – Growing mediums include soils, peat and other organic materials, gravel, and other aggregates

Vegetation – Plants used in green roof applications must be easy to maintain and tolerant of extreme weather conditions including heat, freezing, and drought, and must have relatively shallow, fibrous root systems. The plants should also be resistant to diseases and insects, and not generate airborne seeds in order to protect surrounding plantings. Climate-appropriate succulents, mosses, and grasses are often best suited for extensive green roof systems. These types of plants are available in a variety of colors, in both deciduous and evergreen options. Many nurseries throughout the country specialize in vegetation for green roofs.

What are the types of Green Roofs?

Green roof systems are often broken down into two types—extensive and intensive systems.

Extensive systems:

Consist of low-lying plants such as succulents, mosses, and grasses

require relatively thin layers of soil (1-6 inches), and plants usually produce a few inches of foliage.

weigh 10-50 pounds per square foot on average

typically accessible only for routine maintenance

most common for residential applications

Intensive systems:

feature deeper soil and can support larger plants including crops, shrubs, and trees

harder to maintain, depending on the plants used

weigh from 80 to more than 120 pounds per square foot

typically designed to be accessible to building inhabitants for relaxation and/or harvesting

How difficult are they to install?

Green roof systems can be implemented in new and existing construction. The roof’s structure must be carefully considered to accommodate the additional loads. Roofs do not need to be flat to support green roof systems, but different systems have varying pitch recommendations and limitations, which should be considered during the design phase. The systems also require selection of appropriate plantings for the climatic region. Flood testing of the roof membrane should also be conducted prior to placement of the green roof system.

What are the costs?

Costs for research, design, and materials of the green roof system and structural support are higher than a conventional roofing system. Extensive systems can cost as little as $7 a square foot, though ranges tend to be $10-15 for extensive, and $15-25 per square foot for intensive systems.

There will be some additional costs involved with maintaining the roof top plantings, but overall maintenance of the roofing membrane will be reduced. Since planted roof systems increase the life-span of the roof, repairs and replacement should be minimized.

5 Responses to “Green Roofs – Digging Deeper”

I am the science director at Swampscott High School and I teach Environmental Science. Lisa Kawski of lmk interiors, ltd. [design@lmkinteriorsltd.com] sent me this link. If you have any informationt that could be used in the classroom I would be interested in reviewing it.

A lot of clients that I talk to want green roofs but don’t understand the fact that they might have to restructure the roof. I am all for green roofs but people should go for basic energy improvements first. a great alternative is to replace the typical Kentucky Bluegrass with native vegetation (prairie grass). Also, one to consider about green roofs is that they extend the lifetime of the roof by at least 15 years.